Chemical recycling of nylon 6 using ionic liquids: From solvent screening to techno-economic assessment

To mitigate plastic waste and advance a circular economy, ionic liquids (ILs) are promising solvents for chemical recycling of polymers. Despite repeated calls for integrated assessments of IL-based recycling systems, such studies remain scarce. Here, we report the first conceptual process design and techno-economic analysis (TEA) of IL-based remonomerization of polyamide 6 (PA6) to ε-caprolactam, together with the first comprehensive solvent screening for purifying and extracting caprolactam from the hydrophilic phase, showing that economic viability can be achieved under the defined assumptions. In the benchmark process using ethyl acetate as the extraction solvent, IL recovery is insufficient for profitability, as confirmed by our sensitivity analysis. To address this, we screened more than 900 solvents using a graph neural network (GNN), followed by cross-validation of top candidates through COSMO-RS and UNIFAC, and subsequent experimental validation. This approach yielded a comprehensive ranking, with 1-octanol, 2-octanol, 1-heptanol, 2-octanone, and pentyl acetate emerging as greener, higher-performing alternatives to conventional solvents. The redesigned, heat-integrated process, featuring reduced IL make-up and 1-octanol-based caprolactam extraction, achieves payback within three years under base-case assumptions and within eight years under a conservative uncertainty bound. If the IL recovery is limited to 99.5 %, the IL price must remain below $14.9 per kg to maintain economic viability. Even with ±10 % variations in other uncertain parameters, the 15-year net present value remains positive. Since all major profitability drivers, such as IL recovery, PA6 conversion, IL quantity, and IL price, are closely tied to IL properties, systematic IL screening and selection would be essential for further technological advancement. • First TEA and process design of IL-based chemical recycling of nylon to caprolactam. • GNN screened over 900 solvents for liquid-liquid extraction of caprolactam. • 1-Octanol outperforms ethyl acetate as an extraction solvent. • Redesigned and heat-integrated process reaches profitability within 3 years. • Maximum nylon-6 waste price including sorting and pre-purification is $5.9 per kg.